How to fix if VX can not go straight?

1. The wheel(s) can not rotate freely like the picture shows?
There are several possible reasons:
The screw(s) to tighten the motor got loose.
For more and how to fix: Open VT and tighten the screws using a screwdriver.
The Motor bracket(s) got broken like this picture. If it is within Warranty, please ask for a replacement/ free accessory (The bracket in MiniQ Motor Wheel Set with Encoder).
The motor axis and/or its gear box is stuck with some fur for used on carpet. Clear it up please.
More details, some pictures to explain.
2. When I want Vortex to go forward, but it can't, instead, it would always turn left/right. But once I hold up from the table or if I place Vortex upside down on the table, the wheels can run freely as they are supposed to. Reported by dominika.regentova
It is because the wheel is rubbing with the magnet. How to fix: Find a washer to avoid this problem.